1. Field of the Invention
The present invention relates to an alternator in which a rotor, a stator, and a rectifier are cooled by external air sucked in through a suction inlet of a rear bracket by rotation of a fan fixed to the rotor.
2. Description of the Related Art
Conventionally, alternators are known that include: a case constituted by a rear bracket and a front bracket; a shaft having a first end portion disposed in a rear bearing housing portion of the rear bracket and a second end portion rotatably supported in a front bearing disposed in a front bearing housing portion of the front bracket; a rotor fixed to the shaft; a stator constituted by: a stator core fixed to an inner wall surface of the case; and a stator coil configured by winding conducting wire onto the stator core; a rectifier disposed inside the rear bracket and electrically connected to the stator coil so as to convert alternating current generated in the stator coil into a direct current; and fans fixed to the rotor, the fans cooling the rectifier by sucking in external air through suction inlets in the rear bracket when the rotor rotates, wherein the rectifier has: a heat sink having heat sink fins standing on a rear surface; and a plurality of diodes fixed to a front surface of the heat sink; and a plurality of rear bearing housing portion fins formed on outer peripheral portions of the rear bearing housing portion face the heat sink fins. (See Patent Literature 1, for example.)
In the case of such automotive alternators, as output increases, the amount of heat generated by the diodes and the stator increases, thereby increasing the temperature of the diodes and the temperature of the stator and also increasing the temperature of the rear bracket housing them, and as a result the temperature of the rear bearing housing portion increases.
In high output automotive alternators, at an ambient temperature of 100 degrees Celsius, for example, the temperature of the stator reaches approximately 230 degrees Celsius, the temperature of the diodes approximately 190 degrees Celsius, the temperature of the rear bearing approximately 170 degrees Celsius, and the temperature of the front bearing approximately 160 degrees Celsius.
The reason that the temperature of the rear bearing is higher than that of the front bearing is that the rear bracket receives heat generated in negative-side diodes because the heat sink of the negative-side diodes is in direct contact with the rear bracket.
Patent Literature 1
Japanese Patent Laid-Open No. 2001-45726 (Gazette: FIG. 5)
In such automotive alternators, although the rear bearing housing portion fins are disposed on an outer peripheral portion of the rear bearing housing portion in order to reduce the temperature of the rear bearing, because air passing through the rear bearing housing portion fins has already passed through rear bracket fins mainly cooling the negative-side diodes and positive-side heat sink fins cooling positive-side diodes and been warmed, at an ambient temperature of 100 degrees Celsius, the air is at approximately 140 degrees Celsius when it passes through the rear bearing housing portion fins.
There is a heat tolerance threshold for each of the members, such as 240 degrees Celsius for the stator, 200 degrees Celsius for the diodes, 170 degrees Celsius for the bearings, for example, and there is a risk that the temperature of the rear bearing alone may exceed the heat tolerance threshold.
If such an event occurs, grease in the rear bearing may undergo a heat change, whereby the grease may no longer fulfill its role as a lubricant, the rear bearing may make rattling noises during rotation of the rotor, and although a bearing having an endurance specification of 180 degrees Celsius, for example, can be used in order to prevent that, one problem has been that such measures lead to cost increases.
Automotive alternator output is generally proportional to electric current in the rotor coil wound onto the rotor, and the electric current in the rotor coil is determined by the temperature of the rotor coil. That is to say, a constant voltage is applied between first and second end portions of the rotor coil, and the higher the temperature of the rotor coil, the smaller the electric current in the rotor coil because electrical resistance in the rotor coil increases.
In other words, in order to improve automotive alternator output, it is necessary to increase the electric current value flowing through the rotor coil, and for that purpose it is necessary to reduce the temperature of the rotor coil. The temperature of the rotor coil depends on heat generated in the rotor coil itself, and also on heat received from the case through the rear bearing and the front bearing.
Thus, in order to cool the rotor coil, methods that increase a rate of cooling airflow passing through the rotor coil are also conceivable, but increasing the cooling airflow rate leads directly to an increase in wind noise.